1. Technical Field
The present disclosure relates to a toner, a process cartridge, a developer, and an image forming apparatus.
2. Description of the Related Art
In a typical electrophotographic image forming apparatus, an electrically- or magnetically-formed latent image is visualized with toner. Specifically, in electrophotography, an electrostatic latent image is formed on a photoconductor and then developed into a toner image with toner. The toner image is transferred onto a transfer medium such as paper and then fixed thereon. In fixing the toner image on a transfer medium, heat fixing methods such as heat roller fixing method and heat belt fixing method are widely employed owing to their high energy efficiency.
In recent years, demand for high-speed-printing and energy-saving image forming apparatus is increasing. In accordance with this demand, toner is required to be fixable at much lower temperatures while providing much higher image quality. One approach for achieving low-temperature fixability of toner involves reducing the softening temperature of the binder resin of the toner. However, such a low softening temperature of the binder resin is likely to cause offset phenomenon in which a part of a toner image is adhered to a surface of a fixing member and then retransferred onto a transfer medium in the fixing process. Reducing the softening temperature of the binder resin also reduces heat-resistant storage stability of the toner. As a result, blocking phenomenon in which toner particles fuse together is caused especially in high-temperature environments. In addition, other problems are likely to occur such that toner fuses to contaminate a developing device or carrier particles, or toner forms its film on a surface of a photoconductor.
As a technique for solving these problems, using crystalline resins for the binder resin of toner is known. Crystalline resins have a property of rapidly softening at the melting point. This property makes it possible to lower fixable temperature of toner without degrading its heat-resistant storage stability at or below the melting point. Namely, it is possible to achieve an excellent balance of low-temperature fixability and heat-resistant storage stability at the same time. Although having high toughness, while at the same time, crystalline resins having a melting point which exhibits low-temperature fixability are plastic deformable due to their softness. The technique of merely using a crystalline resin for the binder resin results in a toner having poor mechanical durability, which causes various problems such as deformation, aggregation, and sticking within image forming apparatus and contamination of image forming members.
In view of this situation, a number of toners using both a crystalline resin and an amorphous resin have been proposed. Such toners are generally superior to those using only an amorphous resin in terms of the balance between low-temperature fixability and heat-resistant storage stability. However, if the crystalline resin is exposed at the surface of toner, the toner particles may aggregate under agitation stress in developing device to cause transfer deficiency, or may contaminate carrier particles and the inside of apparatus. In addition, external additive may be embedded in the surface of toner to degrade chargeability and fluidity of toner. Accordingly, the addition amount of crystalline resin has been limited. It has been difficult to take advantage of crystalline resins.
In addition, a number of toners have been proposed which use a resin in which crystalline segments and amorphous segments are chemically bonded. Moreover, toners using a resin in which crystalline polyester and amorphous polyester are chemically bonded have also been proposed.
Such toners can achieve a good balance between low-temperature fixability and heat-resistant storage stability but their softness arising from the crystalline segment has not basically been remedied. The problem regarding mechanical durability of toner has not been solved by these toners.
A toner using a crystalline resin has another problem of rub resistance of the resulting image. When the toner is once melted by heat to be fixed on a transfer medium, it will take a certain time until the crystalline resin recrystallizes and the surface of the image cannot promptly recover its hardness. As a result, the surface of the image may be scratched upon contact with discharge roller or conveyance members in the paper discharge process after the image fixing process, reducing the gloss of the image.